The European Reference Laboratory of Air Pollution implements the static volumetric method for the preparation of nitrogen monoxide and sulphur dioxide reference standard gas mixtures. According to the new ISO guide for the espression of uncertainty of these standards is up to 0.8% for nitrogen monoxide in the range 100 to 600 ppbv, and up to 0.4% for sulphur dioxide in the range 200 to 400 ppbv. The value presented in the present paper suggest that there is a 95% probability of the true values, the standard procedure for the implementation of the static volumetric method must be rigorously followed, and instruments must be carefully maintained.